(1) Field of the Invention
The invention relates to a seabed conduit support apparatus for the proper installation of, for example, a subsea gas transmission conduit.
(2) Brief Description of the Prior Art
There are many instances in which it is desirable to place a fluid transmitting cylindrical conduit across a portion of a river, stream, ocean or other comparatively large or deep body of water. Such conduits are typically utilized to carry hydrocarbons such as oil or gas, or other fluids.
Such conduits are typically layed on the seabed and extend for many miles. Some such conduits systems have been placed in extreme water depths up to 2000 feet, and in the future will be installed in even greater depths. In certain cases, the seabed contours may not be uniform enough to provide continuous support for a long pipeline. The seabed will typically be uneven, with considerable undulations and irregularities in the seabed structure being frequently found in the area across which a conduit is to be placed. Free spans of such conduits or pipelines will occur where the conduit or pipeline crosses such undulations or irregularities in the seabed.
The flow of bottom currents past a free span may, in turn, cause unsteady flow patterns around the pipeline due to vortex shedding. If the vortex shedding frequency coincides with the natural frequency of the pipeline free span, in-phase or harmonic oscillations will occur. The dynamic stresses caused by these pipe oscillations can result in long term fatigue of the pipeline or conduit parent metal and its welds which, in turn, can substantially reduce operating life of the conduit or pipeline. This problem can be remedied by providing intermediate support to the pipe thereby reducing the unsupported span length.
In the past, the unevenness in the seabed was commonly adjusted for or compensated by the selective placement of cement bags or grout pillows under the pipe. In the case of grout pillows, a rubber-like bladder is positioned in the undulation by a diver, and liquid cement is pumped into the bladder through a hose connected to the bladder and extending to a pumping barge or boat or other element at the surface.
While such approaches have been beneficial in comparatively shallow environments, i.e. depths of 500 feet or less, it has been found to be impractical to incorporate such methods in deeper waters because of the difficulty and lack of safety in using divers at such increased depths. Moreover, special measures are required when using such techniques where variations in slopes on the seabed are encountered.
The present invention addresses the deficiencies of the prior art, as described.